Migrate gatsby-plugin-highlight to monorepo

[ccschmitz]

It's currently in a separate repo, but would be nice to pull into the main repo.

[linear]

HIG-4684 Migrate gatsby-plugin-highlight to monorepo

[greptile-apps]

1. Create a new directory sdk/gatsby-plugin-highlight.
2. Move the plugin code from the separate repo to sdk/gatsby-plugin-highlight.
3. Update sdk/gatsby-plugin-highlight/package.json with the appropriate details.
4. Add the new package to the root package.json workspaces.
5. Update docs-content/getting-started/3_client-sdk/5_gatsbyjs.md if necessary.
6. Ensure highlight.io/components/QuickstartContent/frontend/gatsby.tsx references the new location.

References

/sdk/highlight-react/package.json
/docs-content/getting-started/3_client-sdk/5_gatsbyjs.md
/highlight.io/components/QuickstartContent/frontend/gatsby.tsx

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